A known multibeam antenna system includes an antenna section with a one-dimensional or two-dimensional array of beam elements, each of which can emit and receive radiation. In order to distribute a transmission signal to each of the beam elements, in a manner so that it has respective different phase shifts at the various beam elements, it is known to provide a beam-forming network which includes one or more stripline Rotman lenses.
A known Rotman lens has an electrically insulating layer which is made of a dielectric material, and has a layer of metalization provided on one side of the insulating layer. For reasons discussed later, it is desirable to minimize the size of a Rotman lens. Accordingly, and since the linear dimensions of a Rotman lens can be reduced by a factor which is the square root of the dielectric constant of the insulating layer, the insulating layer in the known lens is selected to have a high dielectric constant.
The metalization on the insulating layer includes an approximately oval-shaped lens portion, a plurality of transmission lines which are each electrically coupled at one end to a first side of the lens portion at spaced locations therealong, and a plurality of bootlace lines which are each electrically coupled at one end to an opposite side of the lens portion at spaced locations therealong. The portions of the transmission lines and bootlace lines immediately adjacent the lens portion taper in width in a direction away from the lens portion, in order to effect impedance matching between the lines and the lens portion.
In this known Rotman lens, the bootlace lines need to have certain lengths in order to effect proper operation of the Rotman lens, in particular so that signals passing therethrough experience a predetermined propagation delay. The bootlace lines are thus often laid out on the insulating layer along a path which is U-shaped or meandering, in order to achieve the desired length. As a result, there is a physical limit to the extent to which the size of the insulating layer of this known Rotman lens can be minimized, because the lens portion takes up a reasonably significant portion of the available space on the insulating layer, and the bootlace lines also take up a reasonably significant portion of this available space. The space taken up by the bootlace lines may be comparable to the space taken up by the lens portion itself.
This presents disadvantages in certain applications, where it is desirable that the Rotman lens be as compact as possible. For example, in a satellite, it is desirable that every component take up the smallest possible amount of space. As another example, a multibeam antenna may be provided on the wing of an airplane, where space is limited and a small size for a Rotman lens is highly desirable.
A further consideration, to minimize the size of the lens portion of a Rotman lens, is to fabricate the lens portion on an insulating layer which has a high relative dielectric constant in the range of about 2.5 to 300. Consequently, the insulating layer of the known Rotman lens is invariably selected to have a high dielectric constant. However, as the dielectric constant of the insulating material is increased, the width of the transmission lines and bootlace lines must be decreased in order to maintain a selected impedance characteristic, which may be 50 ohms. As a practical matter, however, the fabrication of narrow lines can present some manufacturing difficulties. Therefore, it is desirable to fabricate the transmission lines and bootlace lines on an insulating layer which has a low relative dielectric constant in the range of about 2 to 4.
Yet another consideration is that it is sometimes desirable to integrate into the known Rotman lens an arrangement commonly known as a Wilkinson combiner. However, since it is easier to fabricate a Wilkinson combiner on an insulating layer with a low dielectric constant than on an insulating layer with a high dielectric constant, because fabrication of the combiner is limited by its dimensions with a high dielectric constant, integration of a Wilkinson combiner into the known Rotman lens can present manufacturing difficulties.
One technique for reducing the size of a known Rotman lens is to physically fold it, but this has not been satisfactory in all respects.